Posted
by
Soulskill
on Saturday November 07, 2009 @08:15AM
from the doc-brown-can-now-power-his-warp-drive dept.

AMESN writes "The Fermi Gamma-ray Space Telescope, launched last year, detects gamma rays from light years away, but recently it detected gamma rays from lightning on Earth. And the energy of the gamma rays is specific to the decay of positrons, which are the antimatter flavor of electrons. Finding antimatter in lightning surprised researchers and suggests the electric field of the lightning somehow got reversed."

The decay of positrons in the largescale discharge of electronic particles may very well lead to gamma ray emissions, however it is crucial to understand the energy output required to reverse the polarity of the discharge so that we can reproduce the phenomenon in a controlled laboratory.

Many of us wish we could forget ever having been exposed to that show as children. I watched an episode on TV Land a few years ago wondering if it really was as good as I thought it was when I was a toddler. Um, no. It ranks down close to the star wars holiday special. Thankfully I grew out of childhood early enough to avoid having ever developed twisted fond memories of Barney, or even Elmo (Elmo ruined Sesame Street).

Another show I am morbidly curious about is "Wizards and Warriors" - is it as horribly b

Anyone who watches Star Trek knows that any time you want to solve technical problems or achieve new developments, all you need to do is reverse the polarity or invert the phase. Why didn't the folks behind the LHC try this? It's have saved billions of dollars and years of delays!;)

If a small roll of Sellotape can generate X-rays in a vacuum chambers, we just need a larger or stickier roll to generate these gamma-rays and anti-matter. Alternatively, get someone to pull the tape extra fast.

Now the monster of frankenstein (powered by lightning) was in fact the first asimovian positronic robot (ok, the alpha one, without any law). With that much discussion about who could be the author to write Asimov's stories, maybe the original Mary Shelley could be the one worthy for that task.

Apparently they've detected gamma ray energies up to 20 MeV from thunderstorms, so given that amount of energy involved I wouldn't think it's that surprising that electron-positron pairs might be created in the process since an electron only has a mass of.511 MeV. The thunderstorms are basically operating like natural linear accelerators.

There already is an efficient way to produce positrons. Bombard a target with any charged particle with kinetic energy greater than 1.022 MeV. Which appears to be what happens in a lightning bolt. This appears to be surprising only to journalists and slashdot readers.

Antimatter in lightning does not suggest that the electric field got reversed; that's nonsense. The electric field is a vector, and it can point in any direction.

What it does suggest is either that the few positrons created or brought by cosmic rays are somehow concentrated by lightning, or that the strong electric fields in lightning are actually pulling a few positron-electron pairs out of the quantum electrodynamic vacuum. The first explanation is probably ruled out unless positron decay gamma rays are also seen all over in the atmosphere, just not as densely concentrated as in lightning.

The second explanation is perfectly possible, if the electric fields in lightning are simply strong enough over large enough volumes of space. Any potential difference greater than 2 m c^2/e will in theory produce positrons and electrons from nothing; this is called 'the Schwinger Effect'. But the rate is ridiculously low unless the field is enormous, and it has not yet been observed. Relatively straightforward calculations would allow one to estimate what sort of electric fields lightning would need to involve, for the observations to be due to the Schwinger Effect.

It's always been my understanding (well, since I was a pre-teen or so) that lightning is not a one-way process. My understanding is that the current flow does indeed reverse several times during a strike, that it's A/C and not D/C. Commentary?

My understanding is that the current flow does indeed reverse several times during a strike, that it's A/C and not D/C. Commentary?

Yes and not really, respectively. There are at least two transfers of charge, one in each direction, but they're by and large discreet events with current flowing one way. So, "alternating" in a sense, but each stroke is best understood as a DC event.

Researchers have been looking for the tell-tale 0.511 MeV photons for decades in lightning storms. The idea is that a lightning channel could act like a natural particle accelerator. So electron-positron pairs could be created. But they have never been seen before from what I understand. But maybe these particles were created in much larger lightning bolts or perhaps the emissions are preferentially directed upwards into space... dunno. Very interesting.

Now, not all of these reports include a positron annihilation signature at 511 KeV. But, 511 KeV emissions were explicitly reported from lightning in the 1970's [nature.com]. And, considering that lightning / thunderstorm related gamma rays are routinely observed with energies up to 10 MeV, there is plenty of energy to create positrons, and so I wouldn't be surprised if all of these reports included the positron annihilation line (or, at least the ones with sensitivity in that energy range).

And, considering that lightning / thunderstorm related gamma rays are routinely observed with energies up to 10 MeV, there is plenty of energy to create positrons, and so I wouldn't be surprised if all of these reports included the positron annihilation line (or, at least the ones with sensitivity in that energy range).

Considering that pair production starts becoming significant at gamma energies above 5 MeV (threshold 1.022 MeV), I would be very surprised if there weren't some 0.511 MeV gammas from thunderstorms. It is also likely that the positrons could be formed by interaction between high energy electrons and matter.

I would think that the gammas are produced in conjunction with sprites (cloud to ionosphere) rather than normal cloud to ground strokes.

The lightning associated gamma rays can be inferred as due to bremsstrahlung associated with electrons released moments after the return stroke and the likely radiation associated with radioactive decay products in the interactions of protons generated in the lightning with the atmospheric constituents

well to start with electrons and protons don't destroy each other. if they were somehow forced together and you threw in an anti-neutrino maybe you could force the reverse of a neutron decay and make them into a neutron, but first you would need to figure out how to force them all together and how to convince the quarks involved to shuffle identities.

so if it's even possible within the laws of physics it's probably at least a thousand years before we can do anything like that, and i don't see any reason t

You do get radiation, if things are energetic enough. If the electron becomes bound to the proton, you get emissions at one of the Hydrogen lines.

If, for example, the electron went all the way to the Hydrogen ground state, you would have emissions at the limit of the Lyman Series [wikipedia.org], up in the hard UV at 91 nanometers.

If things are more energetic, you will get electrons and protons combining to form free neutrons. These will decay [gsu.edu] (this decay is called beta decay) and release gamma rays at 782 KeV, but since the half life of free neutrons is 10.3 minutes, this will be really spread out in time and hard to see. Free neutrons have been directly detected from lightning strikes, so some of this is presumably going on.

Informative, my ass. The collision of an electron with a positron with a collision energy greater than 1.022 MeV could result in the creation of one or more electron-positron pairs. The positrons would then collide with an electron, and the annihilation would release a pair of 511keV photons. It's certainly not the only possible result of such a collision, but I'm not teaching a physics course here, so I won't give you a list of the possible products at various energies.

According to Wikipedia: [wikipedia.org] "Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It has been proposed that essentially all lightning is triggered through a relativistic process, "runaway breakdown" [wikipedia.org], seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms."

Does anyone know what the cross section of a lightning bolt looks like? I've always wondered if forces akin to the skin-effect are trying to spread out the electrons while it's constrained in a tube of plasma. Is it round? Is it a sheet? What's the electron density like? What sorts of pressures would you expect in the center of a bolt?

Just curious... but I'm unable to find a google hit and too dumb to simulate it.

> Does anyone know what the cross section of a lightning bolt looks like? I've> always wondered if forces akin to the skin-effect are trying to spread out> the electrons while it's constrained in a tube of plasma.

Other way around. The magnetic field generated by the current pinches it.

The acoustics of thunder offer some insight into this. It definitely has area to its cross-section or thunder would be much quieter and bi-directional instead of omnidirectional. The pressure is comparatively quantifiable by measuring the SPL of the thunder from a distance, apply inverse square law, etc. Perhaps you can get to electron density from there.

The shape of the cross-section of lightning could be measured reasonably accurately by recording a thunderstorm from multiple locations and determining

I expect the cross-section would be for the mostpart circular, since there is no conductive conduit apart from the air and rain. The electrons will want to be away from each other, but need to be close enough to energize the air to become a conductive conduit instead of a dielectric. Same reason atomic electron orbits are spherical.

The acoustics of nuclear physics... *flexes nerd muscles*

Agreed.. what I wonder though is do you end up with a tube of electrons surrounding a vacuum, or a more uniform distribution of electrons. What is the environment like that's created inside the plasma, and what happens to other high energy particles, say cosmic rays, that enter this region?

what I wonder though is do you end up with a tube of electrons surrounding a vacuum, or a more uniform distribution of electrons. What is the environment like that's created inside the plasma, and what happens to other high energy particles, say cosmic rays, that enter this region?

I think we have a pretty good understanding of plasma. Just look at a florescent or neon lamp.
A lightening bolt is a tube of mostly positively-charged nitrogen ions in a cloud of electrons. The super-heated gas glows brightly as

According to the wikipedia article on lightning, the average length of a strike is 30 microseconds. A 30 microsecond pulse of tens to hundreds of thousands of amps should definitely NOT be considered DC. You're correct that you're not dealing with a repetitive alternating current flow, but it is far, far removed from the quiescent state used when performing DC analysis. The change in current, and hence the magnetic field generated, is absolutely immense.

Definitely a tube surrounding a vacuum. Considering the relatively small amount of space and the volume of thunder produced, we're definitely talking about the instantaneous evacuation and 'revacuation' of the entire bolt of lightning. I don't even need an SPL meter for that.

Just to steer the conversation back on topic, your harmonic subspace bubble isn't going to do jack to protect you from the gamma-ray discharge from an antimatter/matter conversion on it's perimeter. In fact it might cause a toroid-effect and trap the gamma-rays inside your shields, interfere with your sensors and might even take some crucial subsystems offline.

Only a combined strategy of cryptographic spread-spectrum modulation of shields combined with aggressive targetted tractor beam vectoring can keep you safe from localized radiation effects and energy weapons. You really need to keep abreast of the technology in these matters or you could leave yourself, your crew, and very valuable data and equipment at risk.

Umm... This would be hydrogen + hydrogen = deuterium + positron? That makes sense... Though to be generating enough positrons to show detectable levels of gammas from space, that would be a huge discovery.

If they're detecting 511KeV gammas generated by "the direct conversion of electrons to energy" not involving positrons, then, yeah, it would be a hell of a discovery, seeing as how it would blow away all those stodgy conservation laws and symmetries and whatnot.

Easy way to get that reaction and only that reaction would be to use only He3 as the fuel source. No other fuel means no other reactions.

As to "All current fusion designs", that's because we're still trying to make it work in the first place and neutron producing reactions are easier to start with right now. If they could eliminate neutrons from the process now, they would, as it would increase reactor life. Also, energy is not purely extracted by neutrons. If it was, the plasma would heat up indefini

Well, some of the polywell/Farnsworth enthusiasts hope to harness boron-11/proton fusion. In the most common case, that produces three energetic He nuclei (alpha particles), each carrying two positive charges at several MeV. Surround the reaction zone with collector plates, and you extract the energy directly as high-voltage, low-current DC.

Boiling water isn't a direct electrical source either, it's all about energy conversion. The easiest way to convert heat into electricity is to first convert it to kinetic energy. The easiest, cheapest, and safest way to convert heat energy to kinetic energy is to boil water and creat a pressure differential to drive a piston or turbine or what have you. It's very effective, and there isn't any compound likely to do the job better than H2O that isn't also prohibitively expensive.

Heat is the easiest form of raw energy to produce, and if boiling water is the easiest, cheapest, and safest way to convert heat energy into kinetic energy (which is then trivial to convert to electrical energy at very high conversion rates).

Heat engines are also still the most efficient form of energy conversion available to us. A typical modern steam turbine generator will convert close to 50% of the heat energy to electricity, and in some applications can convert as much as 90%. Combustion engines are typically in the 30% range, but getting higher, though they have a theoretical hard limit at 37%. Photvoltaic is coming along, but frankly it's still young and the readily available PV cells compare poorly to combustion and turbine engines. The theoretical limit for a single cell is about 40% efficiency (with light concentrators), but new techniques are working around that limit (they use multiple materials in the cell, effectively combining several cells in one) and the current record is around 43%.

The big problem PV has vs combustion or turbine engines is energy density - the fuel sources the later two methods use are significantly more energy dense than plain sunlight. Sunlight throws a lot of energy everywhere, but only a little in any particular spot. Concentrating it effectively has always been a problem.

Fusion releases the binding energy of the fused nucleus. It's like what happens in a chemical reaction except the mediating force is the (residual) strong force instead of the electromagnetic force, so you tend to get more energy. That amount of energy is still very small compared to the total energy represented by the mass of the reactants. It's not really matter to energy power generation because the mass that gets converted to energy is not really "matter" but rather potential energy. You're talking

During its first 14 months of operation, the flying observatory has detected 17 gamma-ray flashes associated with terrestrial storms -- and some of those flashes have contained a surprising signature of antimatter.

In other words, they have detected 17 gamma-ray flashes due to lightning, and some of them have the signature of antimatter (i.e. the electron-positron annihilation).

I'm not sure how that's not exactly what you're saying they didn't say. Just because they didn't say 511 KeV? If 511 KeV is the signature of electron-positron antimatter collitions, and they've found the signature of antimatter collisions in some (not all) of the storms, wouldn't that suggest they are seeing 511 KeV bursts?

Here's more:

During two recent lightning storms, Fermi recorded gamma-ray emissions of a particular energy that could have been produced only by the decay of energetic positrons, the antimatter equivalent of electrons.

It seems pretty specific about what they are seeing, it is simply stated in a high-level language that the common interested-but-not-knowledgeable reader can understand.

This is essentially an online science news magazine, not a journal for published papers seeking peer review. They are only going to give you the gist of the information at a high-level, and from there if you have better knowledge of the subject you should have an automatic deeper insight into what they might be seeing.

It's not like it's some amature job either, the space telescope was built to find this sort of thing, so finding these signatures is not like some wack job pop-sci company pushing nonsense in a press conference to attract investors before folding in a few years.

Any second year physics major should be able to tell you the electric field required in a plasma for electron positron pair production. The calculation is easy in a near vacuum. At atmospheric pressure it's a bit more difficult, but not that much.

I find it so funny that, everytime a piece of news is posted here about someone discovering something fascinating to the scientific community, there's always a group of people that already knew about it/didn't find the discovery so relevant/etc. These people usually write long posts with technical vocabulary unfolding the misteries of the discovery to everyone...

...and some of us find this one of the most valuable parts of Slashdot. If you want uninformed commentary, there are usually at least a few subthreads full of it on any given topic here, and there are LOTS of other sites better suited for your needs.